In topic 2.4 you bent light through a single flat surface and a slab. Here we put two refracting surfaces together at an angle — and that one change is why a prism does something a slab never can.
1. Present the physical scene
Picture a solid glass prism. Its cross-section is a triangle. Stand it on a table with one sharp edge pointing up and the flat face resting down. The two slanting glass faces — the ones the light will actually pass through — are the refracting surfaces. The angle between those two surfaces, right at the top edge, is the angle of the prism, written . It is also called the refracting angle. The flat bottom is the base.
Now shine one thin ray of one single colour — say red light from a torch through a slit — onto one slanting face. The ray goes into the glass at the first face, travels through the glass, and comes out of the glass at the second face. Two surfaces, so two bendings.
Here is the thing you are going to discover. When that ray finally leaves the prism, it is not pointing the same way it came in. It has been tilted. It comes out bent towards the thick part of the prism — towards the base. That total tilt has a name: the angle of deviation, (the Greek letter delta — it just means "the angle by which the ray got turned aside").
Stop scrolling. Try it in your head before reading on. The ray bends once going in and once coming out. Both bends turn it the same way — towards the base. So does the ray come out parallel to how it went in, or tilted?
(Answer: tilted. Two bends the same way add up — the ray comes out turned towards the base, not parallel to the original direction. That total turn is .)
You can now name the four things on a prism — the refracting surfaces, the angle of the prism , the base, and the angle of deviation — before drawing a single line.